1. Field of the Invention
The present invention relates to a progressive scanning conversion apparatus for converting an interlaced scan video signal into a progressive scan video signal.
2. Description of the Related Art
A conventionally well known progressive scanning conversion apparatus for converting a 2:1 interlaced scan video signal into a 1:1 progressive scan video signal is a motion adaptive line scanning interpolator which is used in a receiving circuit for IDTV (improved definition television). In the motion adaptive line scanning interpolator, a motion detector determines whether there is a motion or not. When there is a motion, a signal from a line scanning interpolator is selected; and when there is no motion, a signal from a field scanning interpolator is selected. As a line scanning interpolator, two types have been proposed: one is a "twice-writing" line scanning interpolator, by which a previously scanned line is written for the second time; and the other is an "average" line scanning interpolator which uses an average of a current line signal and a previous line signal. Further, another type of line scanning interpolator has been proposed, which uses an average of the pixel values in an oblique direction having a high level of correlation (oblique average line scanning interpolation), in order to prevent deterioration in the resolution of an oblique edge (Japanese Laid-Open Patent Publication No. 6-153169).
A conventional progressive scanning conversion apparatus will be described with reference to FIGS. 28, 7 and 8.
FIG. 28 is a block diagram of a conventional motion adaptive line scanning interpolator 100. An interlaced scan signal which is input to an input terminal 2801 (hereinafter, referred to as a "next field signal") is output from a field memory 2802 as a video signal delayed by one field (hereinafter, referred to as a "current field signal") and from a field memory 2803 as a video signal delayed by another field (hereinafter, referred to as a "previous field signal"). Based on the differential absolute value between the next field signal and the previous field signal, whether a motion exists or not is determined pixel by pixel by a motion detector 2805. A line scanning interpolator 2807 generates an interpolated signal by one of two methods: one is the twice-writing line scanning interpolation, by which a video signal which is delayed by a line memory 2804 by one line (hereinafter, referred to as a "previous line signal") is written for the second time; and the other is average line scanning interpolation, by which an average of the previous line signal and the current line signal is used. A field scanning interpolator 2806 generates an interpolated signal from the previous field signal. A switch 2808 selects one of the interpolated signals based on the determination result of the motion detector 2805. If a motion exists, the switch 2808 selects the interpolated signal generated by the line scanning interpolator 2807; and if a motion does not exist, the switch 2808 selects the interpolated signal generated by the field scanning interpolator 2806. The current field signal and the interpolated signal which is output from the switch 2808 are converted into a progressive scan signal by a time axis compressor 2809 and then output from an output terminal 2810.
FIG. 7 is a view showing an oblique edge on the display. Letters a through n denote original pixels on original lines which represent an interlaced scan video signal. Letters p0 through p6 denote interpolated pixels to be generated by interpolation in order to generate a progressive scan video signal. For simplicity, letters a through n and p0 through p6 also represent the values of the respective pixels. The values of the original pixels (corresponding to the luminance on the display) are: a=b=c=d=e=h=i=j=100 and f=g=k=l=m=n=0. The edge parallel to the direction f-k shown in FIG. 7 will be referred to the "f-k" edge. The display is white in an area upper left to the f-k edge and black in an area lower right to the f-k edge. The motion adaptive line scanning interpolator 100 operates in the following manner to generate an interpolated pixel.
In the case where the motion detector 2805 determines that there is no motion, the switch 2808 selects the interpolated signal generated by the field scanning interpolator 2806. If the image is a still picture, the pixel values are: p0=p1=p2=p3=100 and p4=p5=p6=0. Thus, a line is generated by interpolation.
In the case where the motion detector 2805 determines that there is a motion, the switch 2808 selects the interpolated signal generated by the line scanning interpolator 2807. If the line scanning interpolator 2807 performs interpolation by twice-writing line scanning interpolation, a previously scanned line is written for the second time. Thus, the pixel values are: p0=p1=p2=p3=p4=100; and p5=p6=0. In this manner, a line is generated by interpolation. If the line scanning interpolator 2807 performs interpolation by average line scan interpolation, an average of the scan lines which are adjacent to the interpolated line (line to be generated) is used. Thus, the pixel values are: p0=p1=p2=100; p3=p4=50; and p5=p6=0.
In the case where an average of pixel values lined in an oblique direction having a high level of correlation is used for line scanning interpolation (oblique average line scanning interpolation), the differential values between a plurality of pairs of original pixels in the vertical and oblique directions passing the interpolated pixel are compared. The direction in which the pair of pixels have the minimum differential value is regarded as having the highest correlation. The average of the pair of pixels in that direction is used as the interpolated pixel value. The directions compared are the vertical direction, three oblique directions to the right, and three oblique directions to the left. For example, with respect to the interpolated pixel p3, such directions are directions a-n, b-m, c-l, d-k, e-j, f-i, and g-h. The pixel values are: p0=p1=100; p2=100 (c-j or d-i), p3=100 (e-j); p4=0 (f-k); and p5=p6=0.
Such a conventional motion adaptive line scanning interpolator 100 has the problem in that the following quality deterioration occurs at an edge of a moving picture generated because of line interpolation.
In the case where the oblique f-k edge shown in FIG. 7 is obtained by twice-writing line scanning interpolation, the pixel values are: p3=p4=100 (i.e., white). Accordingly, the edge is not completely straight but is zigzagged. As a result, interline flicker and pairing artifact which are generated during interlaced scanning are not alleviated. The interline flicker and pairing artifact are causes of image quality deterioration in an interfaced scan moving image.
In the case where the oblique f-k edge shown in FIG. 7 is obtained by average line scanning interpolation, the pixel values are: p3=p4=50 (i.e., gray). As a result, interline flicker and pairing artifact are slightly alleviated but the resolution in oblique directions is lowered, thereby blurring the oblique f-k edge.
In the case where an average of pixel values located in an oblique direction having a high level of correlation (oblique average line scanning interpolation) is used for line scanning interpolation, the pixel values are: p3=100 and p4=0. The f-k edge is sufficiently generated. In the case of an oblique line A shown in FIG. 8, the values of the pixels p0 through p6 are each 100. The interpolation direction for the pixel p3 cannot be specified since the differential values, namely, the correlation is the same in the directions a-n, b-m, c-l, e-j and g-h. Even if an algorithm for selecting an intermediate direction is used, the value of the pixel p3 is 100 by selecting the direction c-l. In the case of the pixel p4 also, the correlation is the same in the directions c-n, d-m and f-k. Thus, the pixel value p4=100. Accordingly, the oblique line A is cut; that is, interpolation is not performed.
For the oblique line A shown in FIG. 8, the average line scanning interpolation is better than the oblique average line scanning interpolation because the line generated by the former is blurred but not cut.
As is described above, the oblique average line scanning interpolation is effective for generating an edge of an image having a relatively large area but cannot be useful for generating a relatively thin oblique line.